1. Field of the Invention
The present invention relates to an optical measuring method for measuring a position of an edge portion of a workpiece or a position of a stepped edge portion of a workpiece of stepped shape or for measuring a gap or a clearance and a difference in level or a step between the edge portions of adjoining two workpieces.
2. Description of Related Art
In a motor vehicle body there are positions where edge portions of two workpieces lie close to each other. The following are some such examples, i.e., the gap between a front fender "a" and a door b as shown in FIG. 1A, the gap between doors b, b on the front side and on the rear side, respectively, as shown in FIG. 1B, the gap between a side panel c and a roof panel d as shown in FIG. 1C, and the gap between the roof panel d and a tail gate e as shown in FIG. 1D. In order to secure an assembling accuracy of the motor vehicle body, it is necessary to control the gap and the difference in level between edge portions of two adjoining workpieces.
Conventionally, these gaps and the differences in level have been measured in a contact method by using a dial gauge or the like. This method has, however, a disadvantage in that the motor vehicle body may be subjected to scratching.
Further, there is known a method in which is used an optical measuring apparatus having a projector for radiating a slit light on a workpiece and an image sensing device for picturing an optical cutting image drawn by the slit light radiated on the workpiece, and in which are measured the shape and the position of the workpiece from a pictured optical cutting image on a screen of the image sensing device. There is known in Japanese Published Unexamined Patent Application No. 61107/1988 a method of measuring the difference in level between a roof panel and a slide roof by utilizing the above-described method. In this method the difference in level between the roof panel and the slide roof is measured from an amount of deviation between the line on the side of the roof panel and the line on the side of the slide roof in the pictured optical cutting image.
It may also be considered to measure the difference in level between edge portions of adjoining two workpieces by using the above-described optical measuring apparatus. In this case, if an optical cutting image is pictured by radiating the slit light so as to bridge the edge portions of the two workpieces, there will appear on a screen a pictured optical cutting image which terminates at the edge portion of one of the workpieces and a pictured optical cutting image which terminates at the edge portion of the other of the workpieces, provided each of the edge portions is bent at an angle larger than 90.degree. such as the edge portion of the door b, the side panel c, the roof panel d and the tail gate e. It becomes thus possible to measure the gap and the difference in level from the position of an end portion on the side of each of the edge portions of the optical cutting image.
However, the end portion of the pictured optical cutting image becomes tapered arcuate in shape with a curvature corresponding to the bent radius of the edge portion. It is therefore impossible to carry out the measurement by locating the position of the end portion to a single point. Consequently, it is difficult to measure the position of the edge portion at a higher accuracy.
Further, in case of a workpiece of stepped shape having a stepped portion a2 like the front fender "a" which is bent at an angle below 90.degree., there will appear on the screen of the image sensing device a pictured optical cutting image of stepped shape corresponding to the cross-sectional shape of the workpiece. It is therefore possible to measure the position of the stepped edge portion a1 of the workpiece from the position of the bent portion in the pictured optical cutting image.
In this case, however, the bent portion of the pictured optical cutting image also becomes arcuate in shape with a curvature corresponding to the bent radius of the stepped edge portion. It is thus impossible to measure the position of the stepped edge portion by locating the position of the bent portion to a single point on the screen. It is therefore difficult to measure the position of the stepped edge portion at a higher accuracy.
In view of the above-described disadvantages, the present invention has an object of providing an optical measuring method in which the above-described problems can be solved and in which the position of an edge portion of a workpiece or a stepped edge portion of a workpiece of stepped shape can be measured at a higher accuracy and, further, in which the gap and the difference in level between the edge portions can be accurately measured.